1. Field of the Invention
The present invention relates to an automatic loop gain control apparatus for maintaining the loop gain of an automatic control apparatus having a closed loop, and more particularly to an automatic loop gain control apparatus suitable for use in an optical information recording/reproduction apparatus.
2. Description of the Related Art
As information has come to be handled in large amounts in recent years, an optical information recording/reproduction apparatus (hereinafter referred to as an optical disk apparatus) that records or reproduces information on a recording medium by using a light beam has been attracting attention.
In order to record/reproduce information accurately, such an optical disk apparatus comprises a focusing servo system for adjusting the focus state of a light beam, a tracking servo system for adjusting the radiation state of a light beam with respect to the track of the light beam and the like.
In such a focusing servo system and a tracking servo system, since a tracking error with respect to the target position of a light beam for each system must be kept from .+-.1 to 0.1 micron, the open loop gain (hereinafter referred to simply as a loop gain) in the servo system is assumed to be 60 dB in low-frequency bands (15 or 30 Hz) by taking the surface vibration and eccentricity of the disk into consideration. An actuator for object lenses shows a gain peaking phenomenon due secondary resonance in bands above several KHz and due to the actuator's construction, causing a sharp phase degradation.
Therefore, in such a focusing servo system and in such a tracking servo system, it is difficult to obtain a sufficient phase margin and gain margin.
In such a servo control system which cannot obtain a sufficient phase and gain margin, a problem arises in that a variation in the loop gain degrades the stabilization of a system considerably.
Therefore, hitherto, an automatic loop gain control apparatus, such as disclosed in Japanese Patent Laid-Open No. 60-22746 (See FIG. 1), has been provided which suppresses a variation in the loop gain as much as possible.
This automatic loop gain control apparatus is designed to adjust its loop gain on the basis of the variation in the total light quantity of a multisplit sensor contained in a tracking and/or focusing servo control apparatus of an optical disk apparatus.
In the above-described automatic loop gain control apparatus, however, although variations in its loop gain due to changes in the refractive index of a recording medium or due to changes in laser power can be absorbed, the automatic loop gain control apparatus cannot cope with variations in its loop gain due to changes in the force constant of an actuator, changes in the sensitivity of an error detector, changes in its gain caused by circuit elements, or the like.
In order to solve such a problem, the automatic loop gain control apparatus disclosed in the Japanese Patent Laid-Open No. 1-169603 has been proposed. This automatic loop gain control apparatus is constructed as shown in FIG. 2.
A description will now be given specifically of the automatic loop gain control apparatus with reference to FIG. 2.
The automatic loop gain control apparatus adds a reference signal of a predetermined frequency to a closed loop, extracts a reference signal before and after this addition point by means of a band pass filter, adjusts the phase of the extracted respective reference signals, full-wave rectifies respective reference signals by means of an absolute value circuit, following which they are made into average values by means of a low-band pass filter, and inputs the respective signals to input terminals X and Y of a dividing circuit. The dividing circuit performs division in the form of Y/X and adjusts a variable gain amplifier so that the result of the division becomes a predetermined value.
According to this apparatus, changes in the loop gain of a closed loop system are directly measured, on the basis of which a variable gain amplifier is controlled to cause the loop gain to change. Therefore, not only variations in the loop gain resulting from changes in light quantity but also variations in the loop gain due to changes in the sensitivity of respective elements forming the closed loop are absorbed, and a stable servo operation can be performed.
However, since the above-mentioned conventional automatic loop gain control apparatus is formed of analog circuits, it is complex and expensive and its accuracy is not high. Also, the detection of the amplitude of a reference signal is performed by changing it into an average value by using a low-pass filter after full-wave rectification. As a consequence, problems arise in that the amplitude cannot be detected unless at least one cycle of a reference signal is taken in, the detection takes time, and realtime loop gain control cannot be performed.
Thus, there is a need for an automatic loop gain control apparatus which is simple in construction, inexpensive, has high accuracy, and is capable of performing the loop gain control of a servo control system in real time.